Interim Standard IS-95-A (IS-95) has been adopted by the Telecommunications Industry Association for implementing CDMA in a cellular system or personal communication system (PCS). In the CDMA system, a mobile station communicates with any one or more of a plurality of base stations dispersed in a geographic region. Each base station continuously transmits over a Pilot Channel a pilot channel signal having the same spreading code but with a different code phase offset. IS-95 defines the spreading code as a pseudo-random bits (PN) sequence having a period 2.sup.15 chips, and phase offset as a multiple of 64 chips relative a zero-offset pilot PN sequence. Phase offset allows the pilot channel signals to be distinguished from one another. The PN bits ("chips") are generated at a data rate ("chip rate") of 1.23 megabits per second.
CDMA service may not be available in all regions. Thus, when a CDMA-capable mobile station is either turned on or carried into a new region, the mobile station must determine whether or not CDMA service is available. One method of determining the availability of CDMA service is to attempt to acquire a pilot channel signal using an exhaustive scan of the pilot code space for each potential CDMA frequency. A mobile station makes a pilot channel acquisition by acquiring the phase offset of the spreading code of a particular pilot channel signal. If the pilot channel signal can be acquired, IS-95 service is available; otherwise, service is not available.
This pilot acquisition method is suitable in when CDMA service is available on the first frequency attempted. When CDMA service is not available, however, the process of scanning can take as long as 15 seconds per frequency. This problem is compounded by each region having a potential of four or more frequency bands allocated for CDMA service. Consequently, service detection can take a minute or longer. As a result, a user wanting to make a call on the mobile station could wait up to a minute to make the call or to discover that a call can not be made on a CDMA system.
Another method of determining whether CDMA service is available is to detect the chip rate of the composite CDMA signal received by the mobile station rather than acquiring a pilot channel signal. This operation can be done in parallel with the pilot code space scan to quickly establish whether to continue the scan or abort and move to the next frequency, or to try another service such as Advanced Mobil Phone Service. A conventional chip-rate detector comprises a delay-and-multiply circuit that performs an auto-correlation by multiplying the received composite CDMA signal by the conjugate of the composite received CDMA signal delayed by a time T.sub.d. If CDMA service is available, the received signal and the conjugate of the delayed received signal will be correlated, thus the mean output of the delay-and-multiply circuit will be an autocorrelation signal that is periodic with a period equal to the inverse of the chip rate. Conversely, if CDMA service is not available, the mean output of the delay-and-multiply circuit will be the autocorrelation of noise, which is not periodic.
To further refine the detection, a conventional CDMA service detector includes a bandpass filter to filter the output of the delay-and-multiply circuit. The bandpass filter can be a software-implemented digital Fast Fourier Transform (FFT) with its passband centered at the chip rate. Furthermore, an energy measurement circuit measures the energy of the filtered output. The measured energy is compared to a threshold, and, if the measured energy exceeds the threshold, then CDMA service is available. Otherwise, CDMA service is unavailable.
A problem with the conventional CDMA service detector lies in the delay-and-multiply circuit. The performance of the delay-and-multiply circuit depends upon many factors, including the chip waveform, cochannel interference of the multiple pilot channel signals transmitted over the Pilot Channel and the multiple traffic channel signals over the Traffic Channels, multipath propagation, and the delay chosen for the delay-and-multiply circuit.
Some chip waveforms are designed specifically to "hide" the signal, i.e., the delay-and-multiply circuit is not able to provide a high correlation because of the particular waveform, but this is usually not a concern in a cellular system or a PCS. The interference of the multiple pilot channel signals and their multipath components combining at the receiver, however, is a concern.
Because multiple pilot channel signals and multiple traffic channel signals, and their multipath components, combine at the receiver to create a time-varying signal, in the same region where CDMA service is available, the conventional chip-rate detector performance will indicate CDMA service is available at some locations, while at other locations the conventional chip-rate detector will indicate CDMA service is not available. This results in a user's lack of confidence in the determination made by the CDMA service detector.
A need therefore exists for a method of and circuit for detecting the availability of CDMA service that reduces the time to determine whether service is available as compared to the pilot-acquisition method, and is less sensitive to cochannel interference and multipath propagation than the conventional chip-rate detector.